POWER QUALITY AND POWER FACTOR CORRECTION
INTRODUCTION
Since most loads in modern electrical distribution systems are inductive, there is
an ongoing interest in improving power factor. The low power factor of inductive
loads robs a system of capacity and can adversely affect voltage level. As such,
power factor correction through the application of capacitors is widely practiced
at all system voltages. As utilities increase penalties they charge customers for
low power factor, system performance will not be the only consideration. The
installation of power factor correction capacitors improves system performance
and saves money.
A number of manufacturers have catalogs and design manuals to assist in the
application of their products. These publications provide guidance in the
selection and placement of capacitors and discuss general provisions that will
affect the overall performance of the installation.
Although the methodology for applying capacitors is relatively straight forward,
there are a number of influencing factors that must be considered. To ensure
that the capacitor installation does not create more problems than it solves,
consideration must be given to non-linear loads, utility interaction and system
configuration.
PQ PROBLEMS RELATED TO POWER FACTOR CORRECTION
It is ironic to think that as steps are being taken to improve the operating
efficiency at a facility, those very steps may be adversely affecting the facility in
other ways. This is sometimes the case when power factor correction capacitors
are installed at a facility. As an example, general application of capacitors on
motors, when applied without regard to the connected system, can result in the
inadvertent tuning of a system to a dominant harmonic. (The implications of this
are discussed further below).
Although “harmonic problems” are attributed to many power system problems, it
is sometimes overly used. There are other ramifications associated with the use
of power factor correction capacitors such as voltage rise and switching
transients. Each of these power quality concepts will be discussed in turn.
HARMONIC RESONANCE
A common problem that occurs when power factor correction capacitors are
installed on a system is harmonic resonance. When this occurs, the power
system at a facility is tuned to a specific frequency due to a combination of the
system inductance and the added capacitance. The system “resonates” at this
frequency, if there are loads at or near the installation that produce that
harmonic.
When this occurs, the normal flow of harmonic currents, from load to utility
source, is altered. When the currents can flow normally, they combine with other
load currents across the system. If the bulk of those loads are linear, there will
not be a significant percentage of distorted current. However, when the flow is
altered by the installation of capacitors, distortion levels may rise, causing
problems within a plant, at nearby utility customers or at system substations or
currents may flow where they are not desired.
When parallel resonant conditions exist, shunt capacitor banks appear to the
harmonic source as being in parallel with the system source reactance (or short
circuit reactance). When harmonic currents, from the harmonic source, flow
through this high impedance circuit, high harmonic voltages develop. The high
harmonic voltages can result in an overvoltage condition on the capacitors
themselves and/or high voltage distortion.
Overvoltage conditions can exceed the voltage rating of the capacitor and result
in capacitor failure. High voltage distortion can result in the mis-operation or
failure of equipment.
When series resonant conditions occur, the capacitor appears to be in series with
line impedance, as seen from the harmonic source. This presents a low
impedance path to the flow of harmonic currents. Currents, then, will flow on the
system in ways that were unintended. This can result in interference on
communications circuits that may be nearby, excessive voltage distortion at the
capacitors or conductor heating.
If the capacitors are placed at the end of long feeders, harmonic voltage
distortion can occur at the capacitor bank since the bank acts as a ‘sink’ for
harmonic currents originating elsewhere on the system. If the capacitors are
placed on the secondaries of service transformers, the capacitor/transformer
combination can appear like a series tuned filter.
Since this combination behaves like a sharply tuned filter, its resonance at a
significant harmonic would result in a very low impedance path. This would result
in a high voltage distortion on the secondary while the primary distortion would
remain within the limits of IEEE 519.
Capacitors can fail with as little as 10% of fifth harmonic content and this can
take place when there are no other noticeable effects on the system. It has been
estimated that 30-40% of capacitor installations are not fully functional due to
excessive harmonic currents.
In a system that is parallel or series resonant, load has a significant influence on
the harmonic distortion. As the load on the system increases, the overall
damping factor of the circuit increases and the sharpness of the resonance
decreases. When the load decreases, the damping factor decreases and the
sharpness of the resonance increases. The sharpness of the resonance
determines the impedance that is seen by the harmonic currents. Therefore,
harmonic voltage distortion will be worse on lightly loaded systems or when the
system load is mostly motors.
Resonant conditions and the influence of load become particularly important
when a plant is operating from on-site generators. The steady state positive
sequence reactance of a generator is much higher than the utility source
impedance mentioned above. As a result, harmonic currents produce higher
harmonic voltages and overall voltage distortion. Additionally, generator
regulators and control systems are sensitive to distortion on the voltage bus. If
the non-linear load on a plant is a significant percentage of the overall generator
load, the generator may not stay online. Furthermore, high harmonic currents
cause heating in the alternator iron which can lead to premature failure.
SWITCHING TRANSIENTS
As mentioned earlier, capacitors are used at all voltage levels. Utilities install
them at various locations on their transmission and distribution systems for
voltage and VAR support.
When the utility energizes a discharged capacitor, the bus voltage will
momentarily collapse. This occurs because the voltage across a capacitor can
not change instantaneously. This is followed by an oscillatory recovery that lasts
about ½ of a cycle. The overshoot associated with this oscillation can result in a
voltage that has a theoretical peak value of two times the maximum value of the
60Hz sine wave (crest voltage). The same effect can occur when a capacitor is
switched off, if re-strike occurs during the switching operation.
Transients of this magnitude and duration are usually not a problem on the utility
system but they can produce problems at a user facility. Severe over-voltages
can appear on facility capacitors through a phenomenon known as voltage
magnification. The voltage at the end-user capacitor can be greater than the
voltage at the utility capacitor. This translates to a peak voltage with a theoretical
upper value of 400% although this is rarely seen.
The highest transient voltages occur at the low voltage capacitor bank when the
characteristic frequency of the switching transient is nearly equal to the resonant
frequency of the low voltage system and when the switched capacitor is ten or
more times the size of the low voltage capacitor.
The IEEE Standard for Shunt Power Capacitors, ANSI/IEEE Std. 18-1992
specifies that capacitors “may reasonably be expected to withstand” transient
over-voltages from 205% - 354% of rated peak kV (depending on the number of
times a year the over-voltage occurs).
Generally speaking, the voltage magnification will not result in capacitor damage.
The problem that usually occurs is the failure or mis-operation of sensitive loads
in the facility where the low voltage capacitors are installed.
VOLTAGE RISE
At many facilities, fixed capacitors are used to reduce cost. Fixed capacitors are
those that are permanently connected to the load bus and are not switched on
and off as the load changes. When the load on the facility is low, the voltage
may increase due to the capacitor being sized for the higher load.
The limit on steady state voltage is generally taken to be 110% of the rated
voltage. If the voltage is allowed to rise above this point, transformers will
saturate and overheat, mis-operation of equipment may occur and equipment life
will be reduced. If the prevailing bus voltage happens to be high, due to
conditions on the distribution system feeding the facility, the voltage rise would
be added to this already higher voltage. Therefore, system voltage should be
checked when considering voltage rise.
FACILITY SURVEY/NEW CAPACITOR INSTALLATIONS
Capacitor installations are usually straightforward, however, a number of steps
can be taken to ensure that the maximum benefit is derived and there will be no
problems when the capacitors are installed. For example, a comprehensive
facility survey and cost analysis will indicate whether the benefit from the
installation justifies the cost.
Many times, when a decision has been made to install power factor correction
capacitors, the cost analysis has been limited to an examination of the utility bills
and an estimation of the likely savings. In most cases this is probably sufficient.
However, when there are power quality issues to consider, this type of analysis
may not reveal all of the costs.
The presence of non-linear loads, utility capacitors and mis-operating equipment
might indicate that power quality problems exist and could be made worse by
adding capacitors to a system. The true final cost may also include extended
monitoring, an engineering study, relocation of existing capacitors, filter design
and installation, switching equipment and/or follow-up measurements and rework.
To determine what elements may be required, it is best to begin with a facility
survey to identify non-linear loads, the size of the service entrance transformer,
other plant data and utility information. This information, taken together, is
usually sufficient. In some cases, additional information is required which may
involve extended monitoring and/or verification of the system one-line diagram.
The cost analysis would take into consideration the additional requirements and
indicate what the true costs will be.
After data has been collected on the facility, a quick assessment can be made to
determine what level of effort may be required to complete an installation. For a
simple installation, where there are no non-linear loads, the process may be as
simple as sizing the capacitor and having it installed.
IS AN ENGINEERING STUDY REQUIRED?
The following checklist identifies situations where an engineering study is
probably required. This checklist can be used if:
-
capacitors are being added for the first time
capacitors are currently installed and additional capacitors are
being added
capacitors are currently installed and problems are being
encountered.
Are capacitors being added to a system where 20%
of the connected load is harmonic sources?
Have there been unexplained operations of fuses or
other protective devices?
Are measured RMS capacitor currents 135% (or
greater) of rated current?
Have there been any failures of capacitors currently
installed at the facility?
Have there been any instances of swelling or
unusual noises on capacitors currently installed at
the facility?
Have there been unexplained failures or misoperations of sensitive equipment?
Have there been an unusual number of motor
failures or unexplained motor failures?
Has the utility imposed harmonic limits?
Is a plant expansion currently being planned that
might include additional harmonic sources?
Is there on-site generation that will provide power to
a significant number of harmonic sources?
MITIGATION TECHNIQUES
DETUNING
De-tuning a system refers to techniques that are used to change the resonance
point of a system and move it away from significant harmonics. As mentioned
earlier, when shunt power factor correction capacitors are added to a system, the
parallel combination of these capacitors and the system source impedance can
tune the system to resonate at a particular harmonic frequency. This high
impedance path is the source of harmonic voltages when harmonic load current
flows through the system.
One technique used to de-tune a system is to add a reactor to the system.
Harmful resonance conditions are generally between the shunt capacitors and
the source impedance. The reactor is added between the source and the
capacitor bank. An effective way to do this to add the reactor in series with the
capacitor bank to move the system resonance point without tuning the capacitor
to create a filter..
Another method that can be used is to change the size of the capacitor bank
being considered. This is often one of the least expensive options. If the
capacitor can be sized to move the resonance point without impacting other
operational aspects (over/under correction, voltage rise, etc.) there would be no
requirements for other mitigation.
De-tuning can also be accomplished by moving capacitors to a point in the
system with a different short-circuit impedance. This can also be considered if
the installation of a capacitor causes telephone interference problems. In many
cases, the capacitor can not be moved far enough in a plant to make a
difference, however, the technique should not be dismissed outright.
If capacitors are currently installed and problems related to harmonic current
sources have been encountered, it may be cost effective to remove the
capacitors. In this case, a comprehensive cost-benefit analysis must be
performed.
FILTERING
In some situations it may be necessary to install filters to minimize the harmonic
currents that are flowing on a system. Generally, filters provide a low impedance
path to shunt the harmonic currents rather than them flowing back through the
distribution system. Filters also change the system frequency response, most
often, but not always for the better.
Adding a filter creates a sharp parallel resonance point at a frequency below the
filer’s tuned frequency. Filters are tuned slightly below the harmonic in case
there is a change to the system or there is a component failure, either of which
might move the resonance point into the filter. Filters typically cost about three
times what a simple capacitor installation might cost.
Filters are usually applied close to the component in a system where there is
significant generation of harmonic currents. These filters are typically tuned to
the fifth harmonic, for three phase loads, and the third harmonic for single-phase
loads. These frequencies represent the lowest harmonic usually encountered on
these systems and the first filter in a system should be tuned to the lowest
frequency.
Filter application is not as simple as simple capacitor application. Analysis that
may range in scope from a survey to long term monitoring and computer
modeling may be required.
Filter capacitors are usually wired in a delta configuration on 480-volt systems.
As a result, they are largely ineffective when it becomes necessary to control
third-harmonic currents. If triplen harmonics are determined to be a problem,
other configurations can be used.
Filters should be placed on a bus where the available fault current is expected to
remain constant. Although the notch frequency of the filter will not change, the
system resonance point might move.
Finally, filters must be designed with the capacity of the bus in mind. The filter
can not be sized solely on the load that is producing the harmonic.
GROUPING OF LOADS
In installations where there are several harmonic current sources, it may be
possible to electrically group the loads. As an example, this technique is used
when the sources are 6-pulse motor drives. Groups of 6-pulse drives can be fed
from transformers with different winding configurations. If the loads are
balanced, the fifth and seventh harmonics tend to cancel with the net profile
being closer to that of a 12-pulse drive.
In a configuration such as this, the lowest harmonic would be the eleventh or
thirteenth. This not only moves the predominant harmonic away from typical
resonance points but it results in higher frequency harmonics. High frequency
harmonics do not have enough energy to damage a system, as would a low
order harmonic.
In addition to grouping loads for transformer feeds, the grouping also allows
some cancellation that naturally occurs from the statistically random nature of
loads and the corresponding harmonic spectrums. Although this cancellation is
not of the magnitude discussed above, it is noteworthy.
EQUIPMENT CHANGES
If it is determined that power factor correction capacitors may affect power quality
at a facility, one solution may be to make some equipment changes. This may
mean replacing some equipment with newer technology equipment or adding
enhancements to that equipment.
If adjustable speed drives were installed without isolation transformers or lineside reactors, consideration can be given to adding the appropriate equipment to
the installation. Transformers and line reactors can provide solutions to a
number of problems.
Line reactors are a cost-effective way to eliminate nuisance tripping of drives due
to the transient over-voltages that result from utility capacitor switching. In
addition, line reactors prolong the current pulse that is typical of the rectifiers on
the input of these drives. This results in a different, and much improved,
harmonic current spectrum. Determining the correct reactor size, for transient
voltage isolation, requires a detailed transient simulation that takes into account
utility capacitor size and transformer rating.
Standard isolation transformers can provide the same sort of transient isolation
but size and cost considerations may preclude this option. Specialized
transformers that provide harmonic mitigation may also be used. Any equipment
added should be installed close to the drive and electrical connections should be
kept as short as possible.
If grouping of 6-pulse adjustable speed drives is not practicable, consideration
may be given to replacing an older technology drive with a newer one or with a
12-pulse unit.
UTILITY SUPPORT
If voltage transients resulting from utility switching are having an effective on
power quality at a facility, consideration can be given to discussing mitigation
techniques with the utility. There are a number of techniques that the utility can
use to minimize these effects.
The most common control techniques are pre-insertion devices and controlled
closing. When pre-insertion is used, a resistive/reactive element is inserted into
the circuit briefly to damp the first peak of the transient. When reactors are used,
they are helpful in limiting the higher frequency components.
Controlled closing involves using a control system to ensure that the capacitor
switching mechanism closes when the voltage on the capacitor closely matches
the system voltage when the contacts mate. This avoids the step voltage that
causes the circuit to oscillate.
Finally, it may be possible to schedule the switching at a time that will have the
least impact on the facility. The timing may be coordinated with the switching of
facility capacitors or the start up of a process. This may involve switching on
capacitors before they are needed but this should not have adverse effect,
particularly if a thorough analysis has been completed.